The present application relates to a battery pack applied to a power source for power tools such as an electric drill, electric screwdriver, and electric disc grinder.
A lithium ion secondary battery (referred to below as a lithium ion battery as appropriate) that relies on doping and undoping of lithium ions is used as a power source (secondary battery) for power tools. The lithium ion battery has a positive electrode obtained by forming, on a positive electrode collector, a positive electrode active material layer that uses lithium composite oxides such as LiCoO2 or LiNiO2 and a negative electrode obtained by forming, on a negative electrode collector, a negative electrode active material layer that uses carbonaceous material such as graphite or non-graphitizable carbonaceous material capable of being doped or undoped with lithium. The positive electrode and negative electrode are laminated alternately with a separator disposed therebetween and bent or wound to be used as battery module. The battery module is housed in a metal can or laminate film together with nonaqueous electrolyte obtained by dissolving lithium salts in aprotic organic solvent to configure a battery.
A protection circuit is necessary to prevent lithium ion batteries from an overcharge, over-discharge, or overcurrent. Since a power tool consumes a relatively large current, protective devices with a high current capacity are necessary. However, FETs (field effect transistor), fuses, and other protective devices with a high current capacity are large in size and expensive. In addition, fuses blow to protect batteries, but they become unavailable again without being replaced.
Japanese Unexamined Patent Application Publication No. 2005-222834 proposes a switch that has an overcurrent protection function in which a mechanical switch and a positive temperature coefficient device (also referred to as a PTC thermistor) are combined. The PTC device has a low resistance at a relatively low temperature, but its resistance increases sharply above a certain temperature.